1. Field of the Invention
The present invention relates to a steering system for vehicles of the type in which a rear wheel is steerable in relation to the steering operation of a front wheel, and more particularly, to a steering system for vehicles in which the steered angle ratio of a rear wheel to a front wheel is variable in accordance with the vehicle speed.
2. Description of Relevant Art
There has already been proposed a steering system for vehicles, in which the steered angle ratio of a rear wheel to a front wheel is variable in accordance with the vehicle speed, and more particularly, in which the rear wheel is steered, at relatively low vehicle speeds, in the opposite direction to the front wheel or substantially at an angle of zero degree and, at relatively high vehicle speeds, in the same direction as the front wheel, as shown in FIG. 6 of the accompanying drawings (Japanese Laid Open Application JP-A-59-26363 dated Feb. 10, 1984).
In such a steering system, as the vehicle speed is decreased with a steering wheel held at a constant steering angle when the vehicle is turning at a high speed in a steering mode in which a rear wheel is steered in the same direction as a front wheel, the steered angle of the rear wheel correspondingly decreases, resulting in a varying difference between the steered angles of the front and rear wheels, whereby the turning radius of the vehicle becomes smaller.
Incidentally, most ordinary vehicles with an unsteerable rear wheel are set so as to have what is called an understeer characteristic which, as the vehicle speed is increased with a steering wheel held at a constant steering angle to raise the stability in high-speed travelling, enlarges the turning radius. Also, in such an ordinary vehicle with an understeer characteristic, the turning radius becomes smaller, as the vehicle speed is decreased while the vehicle is turning.
However, in those vehicles equipped with a steering system in which the steered angle ratio of a rear wheel to a front wheel is variable in accordance with the vehicle speed, such an understeer characteristic appears to be more effective than in the ordinary vehicle in which the rear wheel is unsteerable.
With such point in mind, in order to improve a steering system for vehicles of the above-proposed type, the present applicant has filed on Oct. 17, 1983 a Japanese Patent Application No. SHO58-193661, to thereby provide a steering system for vehicles in which the correction of the steered angle ratio of a rear wheel to a front wheel is permitted only when the vehicle is in a substantially straight travelling condition.
Incidentally, in such steering system, because of the restricted correction of the steered angle ratio of a rear wheel to a front wheel, there may appear a relatively large relative deviation between the steered angle ratio the vehicle actually has and a converted steered angle ratio corresponding to the actual vehicle speed, or in other words, there may be caused a relatively large relative deviation between the vehicle speed the vehicle actually has and a converted vehicle speed corresponding to the actual steered angle ratio.
In this respect, the present invention has been achieved to further improve a steering system for vehicles of the type described above.